This invention relates to an automatic transmission for a vehicle having a counter shaft for transmitting the output from an engine-driven torque convertor to a differential gear.
A vehicle of a front drive type having an automatic transmission is generally provided with a counter shaft between the transmission and the differential gear. The counter shaft is provided with a counter driven gear at one end for receiving an output from the transmission and with a pinion gear at the other end for driving the differential gear. Both of the gears are normally helical gears and as the result, a thrust acts in the axial direction of the counter shaft during the rotation of the counter shaft. For receiving the thrust, the counter shaft is supported by a tapered roller bearing. When the tapered roller bearing is rotated in transmission oil, a pumping action occurs due to a centrifugal force generated by the rotation of the roller of the bearing and thus the transmission oil is pumped from the smaller diameter portion of the roller to the larger diameter portion thereof, and is further supplied from the roller bearing to the counter driven gear. Accordingly, the amount of the transmission oil stirred by the counter driven gear is increased, thereby increasing the amount of air bubbles mixed into the oil and excessively raising the level of the oil surface. When a large number of air bubbles are mixed into the oil, the efficiency of the torque convertor is deteriorated and therefore the efficiency of the automatic transmission is also deteriorated. If such aerated oil is sucked into an oil pump, the pumping efficiency is decreased and the air is likely to dwell in a hydraulic circuit of the automatic transmission, thereby resulting in failure of the hydraulic control.